DHA intake of US adults is low and conversion of ?-linolenic acid (18:3n-3) to DHA (22:6n-3) is also low. As a result US women enter pregnancy with lower DHA status than other countries in the developed world suggesting a potential deficiency of an important nutrient that has been shown to reduce inflammation. The goal of this Phase III Clinical Trial (randomized, double-blind, and placebo-controlled) is to test the primary hypothesis that supplementing US women with 1000 mg of docosahexaenoic acid (DHA), 800 mg above the amount currently in many prenatal nutritional supplements (~200 mg) during the half of pregnancy can reduce early preterm birth (ePTB) (<34 wk. gestation). This was a favorable secondary outcomes of a US trial (RO1 HD047315, Clinical Trials.gov ID: NCT00266825) conducted at one of the proposed study sites on which the current proposal is modeled. Secondary outcomes include birth weight <1500g and pregnancy outcomes (preeclampsia, gestational diabetes, C-section delivery). We propose a Bayesian Adaptive Design with 90% power to detect the hypothesized outcomes with an estimated 938 subjects that allows the study to stop early if strong results (probability >0.995) are observed before the study is scheduled to end and places 60% of the participants on the better DHA dose. Conventional equal randomization with 90% power would require 1200 subjects, be longer, and place only 50% of the participants on the better DHA dose. The proposed study has the potential to determine if nutritional supplementation with DHA during pregnancy can reduce the incidence of ePTB, a serious public health problem with large societal and family costs. The randomization design has the potential to significantly expedite findings to the research community and clinical practice.